(1) Field of the Invention
This invention relates to a preparation process of indoles from their corresponding anilines and diols, and more specifically to a process for preparing indoles from their corresponding anilines and diols in which catalysts, which have been deteriorated through their use in the reactions, are regenerated for their reutilization.
(2) Prior Art of the Invention
Indoles are known as raw materials for the chemical industry and especially, have become important as raw materials for the synthesis of perfumes and fragrant materials as well as amino acids in recent years.
Numerous attempts have been made to date for the synthesis of indoles. However, they were all accompanied by one or more problems such that lots of byproducts were resulted, expensive raw materials were required and/or complex preparation steps were indispensable.
Recently, some catalyst systems effective for reactions, in which indoles are synthesized through fewer steps from economical raw materials, i.e., their corresponding anilines and diols, have been being found. Cu-Cr, Cu-Co, Pd/SiO.sub.2, Pt/SiO.sub.2, CdS and the like may be mentioned for instance. However, these catalyst systems are all accompanied by one or more drawbacks such that they undergo severe reductions in catalytic activity and/or their catalytic activities are by themselves low. Due to these drawbacks, they are not suited as catalysts for actual application. The present inventors made a variety of investigation on the reactions, and have already reported that the performance of the reactions may be improved by adding water in the reaction systems or effecting the reactions under an elevated pressure.
In addition, the present inventors also revealed that catalyst systems, each of which contains any one of Group Ib metals of the periodic table, i.e., Cu, Ag and Au as an effective component, are effective for the preparation of indoles and allow to conduct the reactions for long time.
Even if these catalysts are employed, they undergo imminent deterioration along the passage of time as the reaction time becomes longer, leading to reductions to their catalytic activities and their selectivities for the intended indoles and hence unavoidably reducing the yields of the indoles.
It is therefore important for the industrial practice of the above-mentioned processes to regenerate their catalysts, which have been deteriorated through their use in the reactions, for their reutilization.
Different regeneration methods may generally be applied depending on the properties of catalysts. No suitable regeneration method has however yet been proposed for catalysts deteriorated in the preparation process of indoles from their corresponding anilines and diols, since the preparation process was developed recently.
As a regeneration method for a catalyst deteriorated through its use in a reaction in which an indole is prepared from its corresponding aniline and diol, it is proposed in Japanese Patent Laid-Open No. 133833/1983 to subject the deteriorated catalyst at a temperature in the range of 300.degree. to 500.degree. C. to a heat treatment in a gaseous (preferably, air) atmosphere containing 5% or more oxygen. As will be demonstrated in Comparative Example 1, Run Nos. 7 and 8, the heat treatment in an atmosphere of air was not able to regenerate the catalytic performance to any sufficient degree and was thus improper as a regeneration method.